linux-kernel - Re: [2.6.33-rc5] Weird deadlock when shutting down

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Date:	Thu, 18 Feb 2010 19:45:35 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [2.6.33-rc5] Weird deadlock when shutting down

On Thu, 2010-02-18 at 08:31 -0800, Linus Torvalds wrote:

> > > halt/4071 is trying to acquire lock:
> > >  (s_active){++++.+}, at: [<c0000000001ef868>]
> > > .sysfs_addrm_finish+0x58/0xc0
> > > 
> > > but task is already holding lock:
> > >  (&per_cpu(cpu_policy_rwsem, cpu)){+.+.+.}, at:
> [<c0000000004cd6ac>]
> > > .lock_policy_rwsem_write+0x84/0xf4
> > > 
> > > which lock already depends on the new lock.
> 
> You don't have a full backtrace for these things?

No, it deadlocks right there, unfortunately.

> We've had lots of trouble with the cpu governors, and I suspect the 
> problem isn't new, but the lockdep warning is likely new (see commit 
> 846f99749ab68bbc7f75c74fec305de675b1a1bf: "sysfs: Add lockdep
> annotations 
> for the sysfs active reference").
> 
> So it is likely to be an old issue that (a) now gets warned about and
> (b) might have had timing changes enough to trigger it.

Well, it used to not deadlock and actually shut down the machine :) So
in that sense it's definitely new. It might have printed a lockdep
warning before, which you wouldn't normally see since the machine turns
off right after this.

> I suspect it is G5-specific (or specific to whatever CPU frequency
> code 
> that gets used there), since I think we'd have had lots of reports if
> this 
> happened on x86.

Yeah, that's puzzling me as well.

johannes

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